As a recess provided in a screw head, that is generally depression-like, H, Z and S shape cross recesses defined by JIS B 1012, or other types of cross recesses called Phillips or Posidrive type, etc. have been generally used (Of the cross recesses defined by JIS, the H shape falls under the category of the Phillips type, and the Z shape under that of the Posidrive type). FIG. 1 shows the H shape cross recess defined by JIS B 1012 as a representative of such typical conventional cross recesses.
As obvious from FIG. 1, the typical conventional cross recess 3 has a central portion 4, four blade engaging portions 5, which are portions for engaging with blades of a driver bit, extending outwardly from the central portion 4 at 90 degrees intervals. The four blade engaging portions 5 are extending in a straight line in the radial direction of the screw 1. A standard conventional driver bit 6 (see FIG. 7) for tightening or loosening the screw 1 with such a cross recess 3 has a shape complementary to the recess 3.
Because in the typical conventional cross recess 3, each of both sidewalls 7 and 8 of the blade engaging portions 5 makes a plane, even if the driver bit 6 is properly inserted into the cross recess 3 in axial alignment with the axis of the screw 1, the engagement between the blade engaging portions 5 and blades 9 of the driver bit 6 is not well. Therefore, when trying to turn the screw 1, the so-called cam-out phenomenon, in which the driver bit slips out of the cross recess 3, is likely to occur. Moreover, the driver bit 6 is liable to tilt relative to the screw 1, and such situation is apt to cause the cam-out phenomenon more and more.
Such being the situation, the present applicant previously proposed, in Patent Reference 1, as a screw with a recess that can eliminate such disadvantages of the typical conventional cross recess, a screw “comprising a recess that is generally depression-like and that has blade engaging portions for engaging with blades of a driver bit, wherein a distal portion of at least one of the sidewalls of each of the blade engaging portion bending from a proximal portion of the sidewall away from the opposite sidewall” and driver bit “wherein a distal portion of at least one of side-faces of each of blades bending from a proximal portion of the side-face away from the opposite side-face”.
According to this screw proposed by the present applicant, because the distal portion of at least one of the sidewalls of each of the blade engaging portion bends from the proximal portion of the sidewall away from the opposite sidewall, if the driver bit with blades of a shape complementary to that of the blade engaging portions is used, upon tightening or loosening the screw, the driver bit is hard to tilt relative to the axis of the screw by virtue of the mating of the bent sidewalls of the blade engaging portions of the screw with the bent side-faces of the blades of the driver bit, and the sidewalls of the blade engaging portions bite into the side-faces of the blades, with the former having a shape bent according to the latter, ensuring that the side-faces securely grip the sidewalls and thereby the engagement between the blades and the blade engaging portions is improved. Consequently, the driver bit is hard to cam out of the recess.    Patent Reference 1: Japanese Laid-Open Patent Publication No. 2000-230526